Method and device for driving a metal halide lamp

ABSTRACT

A method for driving a lamp is described includes generating a light controlling DC electric field E with a desired direction and a desired strength inside the lamp such as to obtain a desirable ion distribution inside the lamp. The lamp is supplied with a commutating current with a DC component, having a constant current intensity and a duty cycle differing from 50%. By setting the duty cycle, it is possible to set a certain average value of the lamp current, allowing the efficacy and/or color temperature of the lamp to be influenced.

The present invention relates in general to a method and a device fordriving a gas discharge lamp, specifically a HID lamp, more specificallya metal halide lamp.

Gas discharge lamps are commonly known. In general, they comprise alight transmitting vessel enclosing a discharge chamber in a gastightmanner, an ionizable filling and a pair of electrodes located oppositeeach other in the discharge chamber, each electrode being connected toan associated current conductor which extends from the discharge chamberthrough the lamp vessel to the exterior. During operation, a voltage isapplied across said electrodes, and a gas discharge occurs between saidelectrodes causing a lamp current to flow between the electrodes.Although it is possible to drive an individual lamp within a relativelywide range of operating currents, a lamp is typically designed for beingoperated at a specific lamp voltage and lamp current and thus to consumea specific nominal electric power. At this rated electric power, thelamp will generate a rated amount of light. Since HID lamps are commonlyknown to persons skilled in the art, it is not necessary to discusstheir construction and operation here in more detail.

A high-pressure discharge lamp is typically driven by an electronicballast supplying commutating DC current. An electronic ballast ordriver for such a lamp typically comprises an input for receiving an ACmains voltage, a rectifier for rectifying the AC mains voltage to arectified DC voltage, a DC/DC upconverter for converting the rectifiedmains DC voltage to a higher DC voltage, a downconverter for convertingsaid higher DC voltage to a lower DC voltage (lamp voltage) and a higherDC current (lamp current), and a commutator for regularly changing thedirection of this DC current. The downconverter behaves like a currentsource. Typically, the commutator operates at a frequency in the orderof about 100 Hz. Therefore, in principle, the lamp is operated atconstant current magnitude, the lamp current regularly changing itsdirection within a very brief time (commutating periods) in a symmetricway, i.e. an electrode is operated as a cathode during 50% of eachcurrent period and is operated as anode during the other 50% of eachcurrent period. This mode of operation will be indicated as square wavecurrent operation.

Although many of the aspects of the present invention are alsoapplicable to different lamp types, the present invention relatesspecifically to metal halide lamps with a relative large aspect ratio,i.e. the ratio of length/diameter is larger than 3 or even 4;conventionally, the aspect ratio is typically in the order of 2.

One problem of metal-halide lamps is that their behavior in a horizontalorientation differs from their behavior in a vertical orientation. In ahorizontal orientation, the spatial distribution of the particles isalmost homogeneous. In a vertical orientation, the spatial distributionof the particles is dependent on the location along the axis of thelamp. This phenomenon, indicated as segregation, is caused by physicaleffects like convection and diffusion, both determined by theatmospheric condition within the lamp. The amount of segregation dependson circumstances like pressure and type of material of the ionizablefilling. The segregation effect increases with increasing electrodespacing, i.e. with increasing aspect ratio.

Since, in a metal-halide lamp, the light is produced by the atoms,segregation has the consequence that the light intensity and light coloris not constant anymore along the central axis of the lamp.

It is a general objective of the present invention to be able toinfluence the light-generating capabilities of a metal-halide lamp inits vertical orientation.

More particularly, the present invention aims to influence the efficacyof a metal-halide lamp.

In accordance with one aspect, the present invention aims to counteractthe effect of segregation, thereby ideally keeping the light intensityand light color along the central axis of the lamp as constant aspossible. In accordance with a particular objective, the presentinvention aims to provide a lamp assembly which automatically maintainsa constant efficacy, independent of the lamp orientation.

In accordance with another aspect, the present invention aims toinfluence the intensity and preferably also the color temperature of thelight generated by a metal-halide lamp. In accordance with a particularobjective, the present invention aims to provide a lamp assembly withvariable color temperature, which is capable of varying the colortemperature over a very large range.

The present invention is, inter alia, based on the recognition that thecurrent through the lamp influences the particle distribution. Based onthis insight, according to an important aspect of the present invention,a metal-halide lamp is operated with a commutating current with a DCcomponent. Depending on the lamp type, varying this DC current componentwill vary the efficacy and/or the color temperature.

These and other aspects, features and advantages of the presentinvention will be further explained by the following description withreference to the drawings, in which:

FIG. 1 schematically shows an embodiment of a metal-halide lamp,

FIG. 2 schematically shows a lamp assembly,

FIG. 3 is a graph illustrating the particle distibution along thecentral axis of a lamp in its horizontal orientation,

FIG. 4 is a graph illustrating the particle distibution along thecentral axis of a lamp in its vertical orientation,

FIG. 5 schematically illustrates electrically induced ion drift,

FIG. 6A is a block diagram schematically illustrating an electronicballast,

FIG. 6B is a graph showing lamp current as a function of time,illustrating square wave current operation,

FIG. 6C is a graph showing lamp current as a function of time,illustrating a commutating current with a non-zero average and 50% dutycycle,

FIG. 6D is a graph showing lamp current as a function of time,illustrating a commutating current with a non-zero average and constantcurrent intensity,

FIG. 7 is a block diagram schematically illustrating an embodiment of anelectronic ballast in accordance with the present invention,

FIG. 8 schematically illustrates an embodiment of an orientationdetector,

FIG. 9 is a block diagram schematically illustrating another embodimentof an electronic ballast in accordance with the present invention.

First, the general operation of a metal-halide lamp, and theconsequences of lamp orientation, will be explained with reference toFIGS. 1-4.

FIG. 1 schematically shows a possible embodiment of a metal-halide lamp,generally indicated at reference numeral 1. The lamp 1 comprises a lighttransmissive vessel 2 having, in the embodiment shown, a circularcylindrical shape and having an internal diameter Di; however, othershapes are possible too. Although not essential in the context of thepresent invention, the vessel 2 is preferably made from ceramicmaterial; alternatively, the vessel 2 could be made from quartz. At itslongitudinal ends, the vessel 2 is closed in a gastight manner by plugsor end caps 3, 4 of a compatible material. The vessel 2 and the plugsand/or end caps 3, 4 enclose a discharge chamber 5 having a diameterequal to the internal diameter Di of the vessel 2 and having an axiallength Li determined by the distance between the end caps 3 and 4. Anaspect ratio AR is defined as the ratio Li/Di.

Inside the discharge chamber 5, two electrodes 6, 7 are arranged at amutual distance EA and substantially aligned with the central axis ofthe vessel 2. In a gastight manner, electrode conductors 8, 9 extendfrom the electrodes 6, 7 through the end caps 3, 4, respectively. If theend caps 3, 4 are made from quartz, the conductors 8, 9 may be molteninto the quartz. Typically, the electrodes 6, 7 will be made from amaterial differing from the material of the electrode conductors 8, 9;for example, the electrodes 6, 7 may be made from tungsten.

Inside the discharge vessel 2, i.e. in the discharge chamber 5, anionizable filling is arranged. The filling typically comprises anatmosphere comprising a substantial amount of mercury (Hg). Typically,the atmosphere also comprises elements like xenon (Xe) and/or argon(Ar). In a practical example, where the overall pressure inside thedischarge vessel 2 is in the order of 1-2 atm, argon and xenon may bepresent in the ratio 1:1. In another practical example, where theoverall pressure is in the order of 10-20 atm, the discharge chamber maycontain mercury and a relatively small amount of argon. In thefollowing, said examples of commercially available lamps will beindicated as relatively low-pressure lamp and relatively high pressurelamp, respectively.

The discharge vessel 2 also contains one or more metal-halidesubstances. Although these may comprise bromides or other halides, thesesubstances typically comprise iodides. Typical examples of such possiblesubstances are lithium iodide, cerium iodide, sodium iodide. Othersubstances are possible, too.

The metal halides are provided as a saturated system comprising anexcess amount of salt, such that during operation of the lamp a saltpool of melted salt will be present inside the discharge chamber 5. Inthe following, it will be assumed that the salt pool is located at thelowest location inside the discharge chamber 5.

In operation, a discharge will extend between the electrodes 6, 7. Dueto the high temperature of the discharge, said substances will beionized and will produce light. The color of the light produced isdifferent for different substances; for instance, the light produced bysodium iodide is red while the light produced by cerium iodide is green.Typically, the lamp will contain a mixture of suitable substances, andthe composition of this mixture, i.e. the identity of said substances aswell as their mutual ratio, will be chosen such as to obtain a specificdesired overall color.

FIG. 2 shows the lamp 1 mounted in a bulb or envelope 11 having at oneend thereof a standard lamp connection cap 12, suitable for screwinginto a standard lamp fitting (not shown). The lamp 1 is axially alignedwith the bulb 11. The lamp 1 is supported by two supportive conductors13 and 14, suitably connected to the electrode conductors 8 and 9,respectively, and electrically connected to electrical contacts of thecap 12. The combination of lamp 1 and its surrounding bulb 11 will beindicated hereinafter as lamp assembly 10.

FIG. 2 illustrates the lamp assembly 10 in a horizontal orientation,i.e. the central axis of the discharge vessel 2 is positionedhorizontally. In this orientation, a discharge arc between theelectrodes 6 and 7 will have its arc axis directed horizontally. In thisorientation, the spatial distribution of particles inside the dischargevessel 2, along the central axis thereof, will be substantiallyhomogeneous, as illustrated by the horizontal line H in FIG. 3. FIG. 3is a graph illustrating the partial particle pressure or particleconcentration as a function of the location along the central axis ofthe discharge vessel 2. This location is represented by the horizontalaxis of FIG. 3 where, by way of reference, the position of end caps 3and 4 and electrodes 6 and 7 are indicated. The graph relates only tothe space between the electrodes 6 and 7, i.e. the location of the arc.

Although in practice the composition of the mixture of the ionizablecomponents may vary such that the partial pressure of each individualionizable component may have a different value, this is not representedin FIG. 3. It is noted that, for the present discussion, the exact valueof the partial component pressure is not relevant, therefore thevertical axis of FIG. 3 does not show any scale marks. Only at the levelof the said horizontal line H, the value 100% is marked. This valuecorresponds to the “maximum” value a partial component pressure reachesalong the lamp axis. Thus, since all partial component pressures aresubstantially constant (and therefore equal to the maximum value) alongthe lamp axis, all mutually different partial pressures are representedin FIG. 3 by only one horizontal line H.

It is important to realize that the light-emitting properties of thelamp 1 at a certain location in the lamp depend on the partial pressureof the ionizable components at that certain location. The higher thepartial pressure of a specific component at said certain location, themore light will be produced having the specific spectral propertiescorresponding to this specific component. Thus, if the partial pressureof the components along the central axis of the lamp is constant, asillustrated by line H in FIG. 3, also the light-emissive properties ofthe lamp 1 as a whole are constant along the central axis of the lamp 1,i.e. constant light intensity and constant color.

FIG. 4 illustrates the problems of segregation associated with avertical orientation of the lamp 1. FIG. 4 is comparable to FIG. 3, andby way of reference the horizontal line H corresponding to thehorizontal orientation of the lamp 1 is shown as well. Otherwise, FIG. 4relates to a vertical orientation of the lamp 1, where a burning arcwill have its arc axis directed vertically. In the example shown, it isassumed that second electrode 7 is the lower electrode while firstelectrode 6 is the upper electrode, corresponding to the illustration ofFIG. 1. Curves (A)-(E) show that in this condition the partial particlepressure is not constant but depends on the location. More particularly,the partial particle pressure decreases with increasing verticaldistance from the bottom electrode 7. This phenomenon is a naturalphenomenon, caused by a combination of convection and diffusionoccurring within the discharge chamber 5, as will be clear to a personskilled in the art.

The effect of segregation may be more or less severe, depending on thecircumstances. As a general rule, the effect is more severe as thepressure in the discharge chamber 5 increases. For instance, curve (A)might relate to a relatively low pressure situation in the order of 1-2atm, while curve (E) might relate to a relatively high pressuresituation in the order of 10-20 atm.

Furthermore, the effects of segregation tend to be most noticeable atone end of the lamp (the upper end in the example shown). Close to thelower electrode 7, the particle concentrations are virtually “normal” inthis example, i.e. identical to the horizontal condition, illustrated bythe fact that, at the location of lower electrode 7, all curvesintersect each other at the horizontal line H. At other locations, theparticle concentrations deviate from their value close to the lowerelectrode 7, the deviation increasing with increasing distance from thelower electrode 7, ending at a maximum deviation close to the upperelectrode 6. As a general rule, the effect is more severe as the lengthLi of the discharge chamber 5 increases.

Furthermore, the severity of segregation is not equal for differentelements within the same lamp. For instance, the segregation in the caseof cerium iodide is more severe than the segregation in the case ofsodium iodide, so that curve (B) might be representing cerium iodidewhile curve (A) might be representating sodium iodide. However, thisdoes not necessarily mean that the partial pressure of sodium iodide isalways higher than the partial pressure of cerium iodide.

Segregation affects the efficacy of the lamp 1, since the amount oflight produced within a certain unit of space is proportional to theamount of light-generating particles within such unit of space, as willbe clear to a person skilled in the art. Thus, segregation causes areduction of light output of the lamp as a whole. Also, segregationcauses an uneven distribution of the light intensity along the length ofthe lamp, more particularly, the higher portions of the lamp willproduce less light than the lower portions of the lamp.

The above already applies if a lamp contains only one light-generatingsubstance. In the case of a mixture of substances, the above appliesalso, but to a different extent for the various components in themixture, as explained earlier. Since the overall color impression of thelight produced by the lamp depends on the light contributions from thevarious components of the mixture, segregation causes a change of thecolor of the light produced by the lamp as a whole on the one hand, andon the other hand segregation causes an uneven color distribution alongthe length of the lamp.

This effect will be most noticeable at the upper extremity of the lamp1, while the situation at the lower extremity of the lamp seems normal.As indicated in FIG. 4, at the lower electrode 7 the relative partialpressures of the light-producing components are substantiallycorresponding to the situation of horizontal orientation, and thegenerated light is in conformity with design expectations. In contrast,at the upper electrode 6, the relative partial pressures deviate fromthe situation of horizontal orientation, the extent of deviation beingdifferent for different components. For instance, in the case of a lampcontaining a mixture of sodium iodide and cerium iodide in apredetermined ratio, the amount of reddish light (for instance: curve A)produced by the sodium iodide will, at the upper electrode 6, be reducedbecause of the reduced concentration of sodium atoms near the upperelectrode 6 while also the amount of greenish light (for instance: curveB) produced by the cerium iodide will be reduced because of the reducedconcentration of cerium atoms. Since, at the upper electrode 6, theintensity of reddish light as well as the intensity of greenish lightwill have been reduced, the overall light intensity around the upperelectrode 6 will have been reduced. Since the reduction of greenishlight is more than the reduction of reddish light, the overallimpression of the color of the light produced around the upper electrode6 will have shifted to reddish.

Curves (D) and (E) show that the segregation can be severe to suchextent that a certain amount of space around the upper electrode 6 isvirtually void of any light-producing ions. What remains is a backgroundglow produced by the mercury buffer gas.

The present invention is based on the recognition that an electric fieldcauses ion transport, and as a consequence also a transport of atoms ofthe same element in the opposite direction. This can be schematicallyillustrated as follows. Consider two electrodes 56 and 57 locatedvertically above each other, the upper electrode 56 being chargednegatively with respect to the lower electrode 57, as schematicallyillustrated in FIG. 5. An electric field between these electrodes isindicated by arrows E. A positively charged particle P+ will be subjectto a force pulling it towards the negatively charged upper electrode 56.In an equilibrium state, a cloud 58 of positively charged particles willhave formed near the upper electrode 56, effectively shielding thenegative charge of the upper electrode 56, thereby reducing the electricfield E.

Thus, between these electrodes 56 and 57, an electric field induces ashift of the particle distribution, such that the concentration ofpositive particles close to the negative electrode is increased. As aresult, an axial gradient of particles will be established.

The present invention uses this recognition to manipulate the light(amount and/or color temperature) produced by a metal-halide lamp bymanipulating the particle distribution in the discharge chamber.Specifically, according to the present invention, during operation ofthe lamp 1, the lamp current applied to the lamp electrodes 6, 7 isgiven an average DC current component, selected such that, on average, alight-controlling electric field E with a desired direction and adesired strength is developed between the lamp electrodes 6, 7.

The following description relates to possible embodiments of driverdevices implementing means for applying such a light-controlling voltageto the lamp electrodes 6, 7. Typical examples of preferred applicationsof the above inventive recognition will be discussed later.

As mentioned before, a metal-halide lamp is operated conventionally withcommutating DC current. FIG. 6A is a block diagram schematicallyillustrating a driver device or electronic ballast 60 for driving a lamp1. An electronic ballast or driver 60 typically comprises an input 61for receiving an AC mains voltage, a rectifier 62 for rectifying the ACmains voltage to a rectified DC voltage, a DC/DC upconverter 63 forconverting the rectified mains DC voltage to a higher DC voltage, adownconverter 64 for converting said higher DC voltage to a lower DCvoltage (lamp voltage) and a corresponding DC current (lamp current),and a commutator 65 for regularly changing the direction of this DCcurrent within a very brief time (commutating periods).

Conventionally, a driver 60 is designed such that its output may beconsidered as constituting a current source with alternating currentdirection but constant current magnitude, having a duty cycle of 50%,i.e. the periods of one current direction are equal to the periods ofopposite current direction, such that each electrode is operated as acathode during 50% of each current period and is operated as an anodeduring the other 50% of each current period. FIG. 6B is a graph showingthe lamp current I as a function of time, illustrating this square wavecurrent operation. It is clearly shown that the magnitude of the lampcurrent remains substantially constant (I_(NOM)), but the direction ofthe current is changed on a regular basis, indicated as a change of thesign of the current from positive to negative and vice versa In a fullcurrent period, the current flows from the first electrode 6 to thesecond electrode 7 during 50% of the time (positive current period), andin the opposite direction during the remaining 50% of the time (negativecurrent period). Thus, the average current I_(AV) is zero.

As mentioned above, according to the invention, the lamp current isgiven an average current value I_(AV) differing from zero.

FIG. 6C illustrates one possibility of implementing the presentinvention. In this case, the average current I_(AV) differs from zerobecause the current intensity during the positive current period differsfrom the current intensity during the negative current period. Again,the current has a duty cycle of 50%, i.e. the current flows in onedirection during 50% of the time (+), and in the opposite directionduring the remaining 50% of the time (−), but the current magnitudeduring the positive periods (+) is larger than the current magnitudeduring the negative periods (−). Thus, on average, an average currentI_(AV) flows from the first electrode 6 to the second electrode 7,indicated by the dashed line I_(AV).

However, this type of implementation is not preferred, because the lampcurrent magnitude during the “positive” half of a current period differsfrom the current magnitude during the “negative” half of the currentperiod, i.e. the current intensity is not constant. Since the lightintensity is proportional to the current intensity, this might lead toundesirable flicker of the lamp.

FIG. 6D illustrates a preferred implementation of the present invention,in which this disadvantage is avoided, and which furthermore is easierto implement by means of an appropriate software or hardware adaptationin existing lamp drivers. In this case, the current intensity remainsconstant at all times, but the average current I_(AV) differs from zerobecause the duty cycle differs from 50%. As FIG. 6D clearly shows, the“positive” current magnitude is always equal to the “negative” currentmagnitude, but the “positive” current period (+) lasts longer than the“negative” current period (−). Also in this case, on average, an averagecurrent I_(AV) flows from the first electrode 6 to the second electrode7, indicated by the dashed line I_(AV).

In both cases, said average current I_(AV) will induce a shift of thedistribution of the positive ions towards the upper electrode 6, asdescribed above.

Thus, according to this aspect of the present invention, the driver 60is designed to have a duty cycle differing from 50%. According to apreferred aspect of the present invention, the driver 60 is designed tohave an adaptable duty cycle. In a possible embodiment, the driver 60may be provided with a control input 66 for receiving a duty-cyclecontrol signal S, and may be responsive to a duty-cycle control signal Sreceived at its control input 66 by setting a duty cycle.

In one aspect, the present invention is aimed at solving the problem ofsegregation affecting the properties of a lamp, intended for horizontaloperation, when mounted in a vertical orientation, leading to a reducedefficacy, such as occurs typically in a low pressure lamp (1-2 atm).According to this aspect of the present invention, means are nowprovided which allow a reduction of segregation and hence an improvementof efficacy. It is even possible to provide a lamp system wherein theefficacy of the lamp can be controlled as desired, and can be set to acertain predetermined value, even in different lamp orientations. It iseven possible to provide a lamp system wherein the efficacy isautomatically kept constant, independent of lamp orientation.

It is noted here that a metal-halide lamp contains, during operation, asalt pool at a certain location inside the lamp. This salt pool issubjected to two flows of particles: inflow of particles entering thepool, and outflow of particles leaving the pool. In a steady statecondition, the inflow and the outflow are balanced. If the lamp currentis given an average current component I_(AV) differing from zero, theinflow or the outflow is influenced, depending on the magnitude anddirection of this average current component. In a steady statecondition, a new balance between inflow and outflow will haveestablished, associated with a new particle distribution within thelamp.

The direction of this average current component may deliberately bechosen such as to increase segregation; in that case, the averagecurrent component has shifted the balance such that more particles haveentered the salt pool. However, in a specific implementation of thepresent invention, the direction of the average current component I_(AV)is chosen such as to effectively eliminate or at least reduce thesegregation effects discussed above. To this effect, the ion flow mustbe directed away from the salt pool in order to compensate segregation.In this case, the upper electrode 6 should, on average, be negative withrespect to the lower electrode 7, and the average current I_(AV) isdirected upwards, as indicated in FIG. 1.

Furthermore, for one specific lamp specimen, there will be one specificoptimum electric field corresponding to one specific optimum duty cycle.This optimum duty cycle will be substantially the same for differentlamps of the same type, and this optimum value can be determinedexperimentally by the manufacturer. Thus, it is possible to provide adriving apparatus 60 with a mode selection switch 67 having threepositions U, H, D, for operating a lamp 1 in a HORIZONTAL position (H)or in a vertical position (U, D) respectively corresponding to aspecific electrode (for instance first electrode 6) being UP (U) or DOWN(D).

If the lamp 1 is mounted in a horizontal orientation, the user may setthe mode selection switch 67 to its H-position. The driver 60 isresponsive to this selection by generating a commutating current with a50% duty cycle and constant current intensity.

If the lamp 1 is mounted in a vertical orientation, the user may set themode selection switch 67 to either its U-position or its D-position,depending on which electrode is up and which is down. Usually, this willcorrespond to the lamp bulb 11 being mounted with the cap 12 downwards(lamp bulb “standing”) or with the cap 12 upwards (lamp bulb “hanging”).The driver 60 is responsive to this selection by generating acommutating DC current with a constant current intensity and apredetermined optimum duty cycle.

If the lamp is symmetrical, the segregation in the case of a standingbulb is identical to the segregation in the case of a hanging bulb, andthe light controlling electrical field for the correction in the uporientation can have the same strength but opposite direction incomparison to the light controlling electric field for the downorientation. If the lamp is not symmetrical, those two electricalstrengths may differ from each other.

Let the duration of the periods during which a certain electrode (7; 6)is positive with respect to the other electrode (6; 7) be indicated as(T₇; T₆), respectively; then, the total current period T_(T)=T₇+T₆. Nowa duty cycle D_(U) (corresponding to the case in which the modeselection switch 67 is in its U-position) can be defined as the ratioT₇/T_(T), while a duty cycle D_(D) (corresponding to the case in whichthe mode selection switch 67 is in its D-position) can be defined as theratio T₆/T_(T). In the case of a symmetrical lamp, D_(D)=1−D_(U).

In the embodiment described above, the mode selection switch 67 isuser-controllable. However, the present invention also provides, in apreferred embodiment, a system for generating light by means of ametal-halide lamp, wherein optimum operative conditions comparable tohorizontal operation are set automatically, adapted to the actual lamporientation. This means that the user is not limited by a certainprescribed lamp orientation, but also that the user does not need toselect an optimum operative condition for the lamp driver: whichever theorientation the user desires to arrange the lamp in, the driver isautomatically adapted to operate in an optimum mode.

Such a system 70 is illustrated in FIG. 7. This system 70 comprises alamp driver or ballast 60 as described above with reference to FIGS.6A-D, without, however, the user-controllable mode selection switch 67.The system 70 further comprises a holder 68 for receiving the lamp cap12 of the lamp assembly 10, the holder 68 having contacts connected tooutput terminals of the commutator 65, as is known per se.

The system 70 further comprises a position detector 80 for detecting theactual orientation of the lamp 1, and for providing the control input 66of the lamp driver 60 with a control signal S indicative of suchorientation, whereas the lamp driver 60 is adapted to drive the lampaccording to the optimum operative conditions corresponding to theactual lamp orientation as sensed by the orientation detector. In thisrespect, the responsiveness of the driver 60 is the same as describedabove, as will be clear to a person skilled in the art.

In principle, any detector suitable for generating a detectable signalindicative of an orientation can be used. FIG. 8 illustrates a possibleembodiment of such an orientation detector. In this embodiment, theorientation detector 80 comprises a cylindrical container 81, forinstance made from glass, with a central portion provided with a groove82 having a larger diameter. The container 81 is sealed and contains asmall amount of an electrically conductive liquid 83, for instancemercury. A first pair of contact electrodes 84 is arranged inside thecontainer 81 near a first axial end thereof, connected to a first set ofconductors 85 extending through the wall of the container 81. Similarly,a second pair of electrodes 86 is arranged at the opposite axial end,associated with a second set of conductors 87. A set of two annularelectrodes 88 is arranged in the said groove, connected to a third setof conductors 89 extending through the wall of the container 81.

In FIG. 8, the detector 80 is shown in a horizontal orientation. Theconductive liquid 83 has moved to the lowermost location inside thecontainer 81, which in this case is the said groove 82, and contactsboth central electrodes 88. Thus, an electrically conductive path isformed between the two corresponding conductors 89. Similarly, if thedetector 80 is placed in an upright orientation, the conductive liquid83 contacts the electrodes 84; 86 on either axial end of the cylinder.

Said conductors 85, 87, 89 are coupled to the control input 66 of thedriver 60; thus, the driver 60 receives the detector output signals, andthe driver 60 knows the orientation of the lamp 1 and drives the lampaccordingly.

The orientation detector 80 may be arranged within the bulb 11 of thelamp assembly 10. However, then it is necessary to provide for contactsin the lamp cap 12 for guiding the sensor signals towards the driver.Therefore, preferably, the sensor is associated with the said holder 68for the lamp assembly 10, such a holder necessarily always having thesame orientation as the lamp fitted therein. Then, a fixed connectionbetween sensor and driver is possible.

As explained above, the average current I_(AV) will induce an ion flowdirected away from the salt pool. The higher the average current I_(AV)magnitude, the stronger the ion flow. On the other hand, the salt poolis maintained by a reflow of atoms. Preferably, the average currentI_(AV) magnitude should not be selected too high, because then the saltpool is displaced to a different location and segregation is stimulated,wheter it be in another direction or not.

In another aspect, the invention aims to provide a lamp system withvariable color properties. A driver preferred for such implementation isillustrated in FIG. 9. A system 90 comprises a lamp driver or ballast 60as described above with reference to FIGS. 6A-D, without, however, theuser-controllable mode selection switch 67, which is replaced by acontrol setting device 91, such as for instance a potentiometer,generating a control signal which can be varied continuously within apredetermined range. The control setting device 91 can beuser-controllable, but it can also be a suitably programmed controller.

Again, the driver 60 produces a commutating DC current of constantcurrent intensity, but now the duty cycle D can be varied, eitherdirectly by a user or by a suitably programmed controller, such as tochange the amount of segregation as desired. In principle, the dutycycle D can be varied from 0 to 100%. Herein, the upper electrode 6 canbe made negative with respect to the lower electrode 7 in order toreduce segregation to a desired extent, as described above, but theupper electrode 6 can also be made positive with respect to the lowerelectrode 7 in order to increase segregation and enhance the colorseparation effect or color changing effect.

In this respect it is noted that, in the case of a metal-halide lampwith a relatively high pressure in the order of 10-20 atm, changing thelevel of the average current I_(AV) was surprisingly found to have agreat influence on the color of the light produced.

With such a system, it has become possible to control a lamp such that awell-defined line is travelled in the standard XY-color or chromaticitydiagram. The composition of the salt mixture enables a certain zerocolor point in this diagram to be selected. By varying the averagecurrent I_(AV) of the commutating current (duty cycle), the color pointof the lamp shifts along a line intersecting said zero color point. Theangle of this line depends, inter alia, on the overall lamp pressure andthe amount of mercury in the lamp: in the case of a low pressure lamp(i.e. overall lamp pressure lower than about 3 atm), said line will besubstantially parallel to color isotherms, whereas in the case of a highpressure lamp (i.e. overall lamp pressure higher than about 10 atm),said line will be substantially perpendicular to color isotherms, whichinvolves a large variation in color temperature.

This aspect of the invention can be practiced in a vertical lamporientation as well as in a horizontal lamp orientation. As explainedabove, segregation will occur if a metal-halide lamp is mountedvertically, and this segregation can be reduced or increased by applyinga DC current component. The important feature in this respect is that itis possible to change the particle distribution instantaneously byapplying a DC current component. This feature is not restricted tovertical lamp orientation.

In a horizontal lamp orientation, a salt pool will have formed at acertain location, which, in the case of a symmetrical, long, thin lamp,typically is at one end or both ends of the lamp. As explained earlier,there is a balance between inflow and outflow of particles into and outof the salt pool, corresponding to a certain particle distributioninside the lamp. According to the invention, it is possible to shiftthis particle distribution by applying a DC current component. Thisphenomenon will also be termed “current induced distribution shift”.

In order to obtain a defined initial situation, it is possible tooperate the lamp at DC current (e.g. duty cycle 0%). Then, after sometime, the salt pool will be located at one of the two ends of the lamp;segregation is now at a maximum.

From this starting situation, the segregation can be reduced by raisingthe duty cycle from 0%. With increasing duty cycle, a new balance willestablish between inflow and outflow, the salt pool initially stayingsubstantially in place. The segregation can be eliminated by raising theduty cycle to 50%. A duty cycle of more than 50% leads to an undesiredtransportation of salt.

The duty cycle range between 0% and 50% determines the color range ofthe lamp, attainable by this aspect of the present invention. When theduty cycle is 0%, the light produced by the lamp can be represented by acertain color point in the chromaticity diagram. The exact location ofthis color point, which will also be termed “horizontal zero” colorpoint, depends on the composition of the mixture of elements within thelamp, and can be selected by suitably selecting this composition, aswill be clear to a person skilled in the art. If the duty cycle isincreased, the color point will shift away from the horizontal zerocolor point. An end point is reached when the duty cycle reaches 50%.Thus, the color point will travel a line, hereinafter termed “colorline”, which has one end point defined by the horizontal zero colorpoint and an opposite end point defined by a 50% duty cycle. Thisinvolves a change of the color temperature in the order of between 1500K and 2000 K.

If the initial situation is reversed, i.e. by initially setting the dutycycle to 100%, changing the duty cycle from 100% to 50% will yieldsubstantially the same results.

Thus, for a high-pressure lamp (10-20 atm), it has proved possible tovary the color temperature over a range in the order of 1500-2000 K.

In the case of a lamp with an asymmetric geometry, the salt pool willhave a preferred location, i.e. the coldest place in the lamp, which istypically one end of the lamp. If we assume that this preferred locationcorresponds to the initial position obtained by setting a duty cycle of0%, then it is possible to increase the duty cycle to above 50%, to alimited extent, before transportation of salt occurs. Hence, the colortemperature variation range will be larger.

This range is even wider (2500-3000K) in the case of vertical lamporientation, due to the fact that in such a case the salt pool willtypically be located at one end of the lamp, i.e. the coldest spot ofthe lamp, typically the lower end. In this case, when the same lamp asdiscussed above is turned from a horizontal orientation to a verticalorientation, while the average DC current is zero, segregation may occurand the color temperature may shift. This shift will depend on thecomposition of the mixture of elements within the lamp, and on theamount of segregation. Again, the light produced by the lamp can berepresented by a color point in the chromaticity diagram, which will nowbe termed “vertical zero” color point.

If an average DC current is now added, depending on the direction of theaverage DC current, the segregation will be increased or decreased,while in both cases the color point will shift.

In the case of a vertically burning lamp, the discharge is asymmetricdue to convection. Typically, the temperature at the upper end of thelamp is higher than the temperature at the lower end of the lamp.Therefore, the partial pressures of the salt components can be higherthan the partial pressures just above the salt pool before condensationoccurs. Due to this, and to the fact that this effect does not occur toan equal extent for all salt components, the range from no segregationto maximum segregation can correspond to an additional variation incolor temperature, and the maximum color temperature variation can belarger than in the case when the lamp is mounted horizontally.

Thus, for a high-pressure lamp (10-20 atm) mounted vertically, it hasbeen found possible to obtain a color temperature variation of about2500 to 3000 K.

With such a system, it has also appeared possible to provide amulti-color lamp. To explain this, reference is once again made to FIG.4. As explained earlier, the severity of segregation can be differentfor different substances, and the same applies for the enhancedsegregation as caused by a light controlling average current I_(AV) inaccordance with the invention. Let us assume that curves (A) and (B)represent sodium iodide and cerium iodide, respectively, for a lampoperated with an average current I_(AV) being zero. Let us furtherassume that curves (C) and (D) represent sodium iodide and ceriumiodide, respectively, for the same lamp that is now operated with anaverage current I_(AV) having a direction selected such as to enhancethe segregation. Then, the lamp will show three bands of light. In thelowest region I of the lamp, the color of the light will be close to“normal” white light, although shifted to reddish. In a second region IIof the lamp, above the first region, cerium iodide is almost completelyabsent, and the emitted light does not have any green contribution fromcerium iodide any more: the color of the light is now completelydetermined by sodium iodide, i.e. red. In a third region III of thelamp, above the second region, also the sodium iodide is almostcompletely absent, and the emitted light does not have any redcontribution from sodium iodide any more; if the lamp does not containany other salts, this third region will emit a blue-ish glow from themercury buffer gas. If the lamp does have a third salt component withless segregation, the light generated by this third component willdominate.

Thus, a very colorful effect of multiple bands with different colors ispossible.

When varying the magnitude of the average current I_(AV), the ion flowdirection may be directed away from the salt pool to decreasesegregation or may be directed towards the salt pool to increasesegregation. In the first case, the magnitude of the average currentI_(AV) should preferably be selected below a threshold level where thesalt pool as a whole is displaced, as mentioned earlier. In the secondcase, the magnitude of the average current I_(AV) does not suffer fromsuch restrictions.

In summary, the present invention thus succeeds in providing a methodfor driving a metal halide lamp 1, the method comprising the step ofgenerating a light controlling DC electric field E with a desireddirection and a desired strength inside the lamp to obtain a desirableion distribution inside the lamp. The lamp is supplied with acommutating current with a DC component, preferably having a constantcurrent intensity and a duty cycle differing from 50%. By setting theduty cycle, it is possible to set a certain average value of the lampcurrent, allowing the efficacy and/or color temperature of the lamp tobe influenced.

Although the present invention has been explained in the foregoing bymeans of descriptions of some exemplary embodiments, it should be clearto a person skilled in the art that the present invention is not limitedto such embodiments; rather, various variations and modifications arepossible within the protective scope of the invention as defined in theappending claims.

For instance, in the above it has been explained that a desirableparticle distribution inside the lamp can be established by causing asuitable light controlling electric field E having a suitable directionand a suitable strength, while further it has been explained thatutilizing a lamp current with a suitable average current intensityI_(AV) is a preferred way of manipulating the particle distribution.However, within the scope of the present invention, other ways ofcausing a suitable light-controlling electric field E are feasible too,such as for instance the use of an external electromagnet.

Furthermore, the description above relates primarily to steady stateoperation. During a start phase of the lamp, the average currentintensity I_(AV) is preferably zero.

1. A driving apparatus for driving a gas discharge lamp, the apparatuscomprising: current generating means for generating a current with asubstantially constant current intensity; commutating means forreceiving said current, and having an output for connecting to the lamp,the commutating means being arranged for commutating said current with aduty cycle differing from 50%; and a mode selection switch having atleast three positions; the commutating means being arranged forgenerating a commutating current with a 50% duty cycle and the constantcurrent intensity when said mode selection switch is placed in a firstposition indicative of a horizontal orientation of the lamp, and forgenerating a commutating DC current with the constant current intensityand a predetermined duty cycle differing from 50% when said modeselection switch is placed in a second position or in a third positionindicative of a vertical orientation of the lamp.
 2. The drivingapparatus according to claim 1, wherein the commutating means arearranged for commutating said current with a variable duty cycle.
 3. Thedriving apparatus according to claim 2, further comprising a controlinput .for receiving a control signal, wherein the driving apparatus isresponsive to the control signal received at the control input to setthe duty cycle accordingly.
 4. The driving apparatus according to claim3, wherein the mode selection switch is coupled to said control input.5. The driving apparatus according to claim 3, adapted for a variablecurrent-controlled particle distribution shift, wherein the drivingapparatus is provided with a control setting device coupled to saidcontrol input; wherein the control setting device is arranged forgenerating the control signal which is continuously variable within apredetermined range; and wherein the driving apparatus is arranged tocontinuously vary the duty cycle of the commutating lamp current inresponse to said control signal.
 6. The driving apparatus according toclaim 1, wherein said predetermined duty cycle has a predetermined firstvalue (D_(D)) differing from 50% when said mode selection switch isplaced in said second position indicative of a standing orientation ofthe lamp, and wherein said predetermined duty cycle has a predeterminedsecond value (D_(U)) differing from 50% when said mode selection switchis placed in said third position indicative of a hanging orientation ofthe lamp; and wherein D_(D)≠D_(U).
 7. The driving apparatus according toclaim 6, designed for driving a symmetrical lamp, whereinD_(D)=100%−D_(U).
 8. The driving apparatus according to claim 1, adaptedto generate said current with a duty cycle equal to 50% during a startphase of the lamp.
 9. A variable color temperature light generatingsystem, further comprising: a driving apparatus according to claim 1,the driving apparatus being capable of driving the gas discharge lampwith a variably settable average current intensity in order to induce avariable current-controlled particle distribution shift in the gasdischarge lamp, so as to allow a color point to travel a color line inthe chromaticity diagram over a relatively large color temperature rangeof more than 1500 K.
 10. The driving apparatus of claim 1, wherein thegas discharge lamp is a high intensity discharge lamp.
 11. The drivingapparatus of claim 1, wherein the gas discharge lamp is a metal halidelamp with an aspect ratio of length/diameter larger than 3 or 4.